1. Feild of the Invention
The present invention relates to a method and apparatus for stabilizing output beam parameters of a gas discharge laser. More particularly, the present invention relates to maintaining an optimal gas mixture composition by performing gas replenishment based on a monitored amplified spontaneous emission (ASE) signal which varies with the gas mixture status.
2. Discussion of the Related Art
Pulsed gas discharge lasers such as excimer and molecular lasers emitting in the deep ultraviolet (DUV) or vacuum ultraviolet (VUV) have become very important for industrial applications such as photolithography. Such lasers generally include a discharge chamber containing two or more gases such as a halogen and one or two rare gases. KrF (248 nm), ArF (193 nm), XeF (350 nm), KrCl (222 nm), XeCl (308 nm), and F.sub.2 (157 nm) lasers are examples.
The efficiencies of excitation of the gas mixtures and various parameters of the output beams of these lasers vary sensitively with the compositions of their gas mixtures. An optimal gas mixture composition for a KrF laser has preferred gas mixture component ratios around .about.0.1% F.sub.2 /.about.1% Kr/.about.98.9% Ne. An F.sub.2 laser has a preferred gas component ratio around .about.0.1% F.sub.2 /.about.99.9% Ne (or He) (see U.S. patent application Ser. No. 09/317,526, which is hereby incorporated by reference). Any deviation from these optimum gas compositions or those for other excimer or molecular lasers would typically result in instabilities or reductions from optimal of one or more output beam parameters such as beam energy, energy stability, temporal pulse width, temporal and spatial coherence, bandwidth, and long and short axial beam profiles and divergences.
Especially important in this regard is the concentration (or partial pressure) of the halogen containing molecules, e.g., F.sub.2 or HCl, in the gas mixture. The depletion of the rare gases, e.g., Kr and Ne for a KrF laser, is low in comparison to that for the F.sub.2, although these rare gases are also replenished at longer intervals.
In industrial applications, it is advantageous to have an excimer or molecular laser capable of operating continuously for long periods of time, i.e., having minimal downtime. It is desired to have an excimer or molecular laser capable of running non-stop year round, or at least having a minimal number and duration of down time periods for scheduled maintenance, while maintaining constant output beam parameters. Uptimes of, e.g., greater than 98% require precise control and stabilization of output beam parameters, which in turn require precise control of the composition of the gas mixture.
Unfortunately, halogen depletion due to electrode erosion occurs in excimer and molecular lasers due to the aggressive nature of the fluorine or chlorine in the gas mixture. The halogen gas is highly reactive and its concentration in the gas mixture decreases as it reacts, leaving traces of contaminants. The halogen gas reacts with materials of the discharge chamber or tube. Moreover, the reactions take place and the gas mixture degrades whether the laser is operating (discharging) or not. The static gas lifetime is about one week for a typical KrF-laser. For a static KrF laser gas mixture, i.e., with no discharge running, increases in the concentrations of HF, O.sub.2, CO.sub.2 and SiF.sub.4 have been observed (see Jurisch et al., below). During operation of a KrF-excimer laser, such contaminants as HF, CF.sub.4, COF.sub.2, SiF.sub.4 have been observed to increase in concentration rapidly (see G. M. Jursich et al., Gas Contaminant Effects in Discharge-Excited KrF Lasers, Applied Optics, Vol. 31, No. 12, pp. 1975-1981 (Apr. 20, 1992)).
One way to effectively reduce this gas degradation is by reducing or eliminating contamination sources within the laser discharge chamber. With this in mind, an all metal, ceramic laser tube has been disclosed (see D. Basting et al., Laserrohr fur halogenhaltige Gasentladungslaser" G 295 20 280.1, Jan. 25, 1995/Apr. 18, 1996 (disclosing the Lambda Physik LP Novatube); and German Gebrauchsmuster DE 297 13 755.7 of Lambda Physik (disclosing an excimer or molecular laser having a window mount with a metal ceiling), each of which is hereby incorporated by reference). Gas purification systems, such as cryogenic gas filters (see U.S. Pat. No. 4,534,034) or electrostatic particle filters (see U.S. Pat. No. 5,586,134) are also being used to extend KrF laser gas lifetimes to 100 million shots before a new fill is advisable. Notwithstanding using improved laser tubes and cryogenic purification systems, the most important techniques for stabilizing gas mixtures include gas replenishment procedures, whereby halogen injections (Hl), rare gas injections, partial gas replacements (PGR) and total pressure adjustments are performed. The specific gas replenishment amounts and intervals are determined based on this evolution of the composition of the gas mixture, which is most prominently determined by depletion of the halogen species. U.S. provisional patent application no. 60/1 24,785 discloses the preferred gas replenishment procedure of the present invention and is hereby incorporated by reference. The '785 provisional application discloses to use several small gas actions such as micro-halogen injections (.mu.Hl) and mini-gas replacements (MGR.sub.i) which are gas replacement procedures performed at multiple carefully selected intervals and amounts, in addition to total pressure adjustments, partial gas replacements (PGR) and new fills.
It is not easy, however, to directly measure the halogen concentration within the laser tube for making rapid online adjustments (see U.S. Pat. No. 5,149,659 (disclosing monitoring chemical reactions in the gas mixture) and Japanese Patent No. JP 10341050 (disclosing a method wherein optical detection of a halogen specific emission is performed)). Therefore, advantageous methods applicable to industrial laser systems include using a known relationship between F.sub.2 or HCl concentration and a laser parameter. In such a method, precise values of the parameter would be directly measured, and the F.sub.2 or HCl concentration would be calculated from those values. In this way, the F.sub.2 concentration may be indirectly monitored.
Previously described methods for laser gas characterization include measuring the spectral width of the laser emission (see U.S. Pat. No. 5,450,436 to Mizoguchi et al.), measuring the spatial beam profile of the laser emission (see U.S. Pat. No. 5,642,374 to Wakabayashi et al.) and measuring other characteristics of the output beam such as bandwidth, coherence, driving voltage or energy stability wherein a rough estimation of the status of the gas mixture may be made (see U.S. Pat. No. 5,440,578 to Sandstrorrr, U.S. Pat. No. 5,887,014 to Das and U.S. patent application Ser. No. 09/167,653 to Vogler et al., filed Oct. 5, 1998).
In the Vogler Application (09/167,653, which is hereby incorporated by reference), a data set of an output parameter such as pulse energy and input parameter such as driving voltage is measured and compared to a stored "master" data set corresponding to an optimal gas composition such as is present in the discharge chamber after a new fill. From a comparison of the data values and/or the slopes of curves generated from the data sets, a present gas mixture status and appropriate gas replenishment procedures, if any, are determined and undertaken to reoptimize the gas mixture. It is desired to improve upon all of the above-described techniques by performing gas control based on the monitored values of a parameter that varies with a known correspondence with the halogen content in the gas mixture. Moreover, the desired parameter would not be greatly influenced by other laser system and output beam parameters, such as resonator alignment, repetition rate, tuning accuracy, thermal conditions, aging of the laser tube and degradation of optics.
Another technique uses a mass spectrometer for precision analysis of the gas mixture composition (see U.S. Pat. No. 5,090,020 to Bedwell). However, a mass spec is an undesirably hefty and costly piece of equipment to incorporate into a continuously operating excimer or molecular laser system such as a KrF, ArF or F.sub.2 laser system which are typical light sources used in microlithographic stepper or scanner systems. Yet another technique measures fluorine concentration in a gas mixture via monitoring chemical reactions (see U.S. Pat. No. 5,149,659 to Hakuta et al.), but this method is not suitable for use with a rapid online correction procedure. It is desired to have a precise technique for monitoring gas mixture status which is easily adaptable with current excimer or molecular laser systems and provides rapid online information.
In typical gas discharge lasers such as excimer or molecular fluorine lasers, a constant laser pulse energy is maintained in short-term notwithstanding the degradation of the gas mixture by regulating the driving voltage applied to the discharge. As mentioned above, long term regulation is achieved by gas replenishment actions such as halogen injections (Hl), total pressure adjustments and partial gas replacement (PGR). The smoothed long-term stabilization of the gas mixture composition uses a regulation loop where input laser system parameter data are processed by a computer (see U.S. patent application Ser. No. 09/167,653 to Vogler et al., above).
In these typical laser systems, an energy detector is used to monitor the energy of the output laser beam. The computer receives the pulse energy data from the energy detector as well as driving voltage information from the electrical pulse power module. This information is not selective enough since the energy monitor signal is influenced not only by the gas but also by resonator optics degradation or misalignment. The typical operation mode is the so-called energy-constant mode where the pulse energy is kept constant by adjusting the driving high voltage of the electrical pulse power module. In this way one gets constant values from the energy monitor. A change of the laser status which again can be caused by gas aging as well as by the status of the laser resonator leads to a change of the driving voltage. It is desired to operate the laser at an approximately constant driving voltage level. To achieve this an appropriate smoothed gas regulation procedure is necessary, In the example of an excimer laser usually the halogen gas component (F.sub.2 in KrF lasers) is depleted whereas the other gases (nobel gases Kr and Ne in KrF lasers) are usually not depleted. Therefore .mu./Hl's or other suitable smoothed gas actions are applied.